It is well known that streamlining the undercarriage of a long vehicle such as a trailer truck or straight van truck will reduce the aerodynamic resistance of a moving vehicle and thus save fuel. However, streamlining the undercarriage of a long vehicle such as a semi-trailer is impractical because integrated enclosures are heavy, costly, easily damaged, interfere with standardized structural design, and prevent access to parts and equipment underneath the vehicle. Consequently, removable attachments have been designed to streamline the undercarriage of long vehicles. However, past designs for removable attachments have failed to achieve significant utilization because they are typically large and heavy enclosures or one piece structures, which are difficult to transport when not mounted on the trailer, difficult to mount, require drilling or welding the undercarriage when mounting, incompatible with different trailer and truck geometries, easily damaged, difficult to repair, become filled with snow and ice in winter driving conditions, do not provide the maximum drag decrease possible, and are too costly to generate a significant return on investment though associated fuel use reductions. The device described herein uses a minimal amount of material resulting in reduced weight and production costs and provides a maximized drag reduction. Prototypes of the device, constructed under U.S. Government contract No. DE-FC36-03G013173 awarded by the Department of Energy, weighed under 150 lbs and reduced a semi-trailer's fuel consumption by 4% in Society of Automotive Engineer's (SAE) J1321 type II standardized fuel economy tests conducted by a respected independent research center. The improved aerodynamic performance of the devise described herein when compared to past designs for aerodynamic attachments, that extend the side wall of the truck or trailer directly toward the ground, is due to the addition of angled flaps directed toward the center of the vehicle. The geometry of the angled flaps was tuned to provide the maximum drag reduction possible by directing crosswind airflow beneath the trailer's rear wheels and axles. The straight side extensions of past designs create low pressure vacuums that tend to direct crosswind airflow into the wheels and axles and can cause large side forces that push the vehicle off its intended course. The angled flaps also provide stiffing means, necessary to prevent the wind induced flapping or oscillations common to the straight side extensions of past designs. Unlike the straight side extensions of past designs, which tend to buckle or collapse in a random fashion if struck by protrusions on the ground (or the ground when the vehicle travels over uneven surfaces) and may sustain damage even if constructed of flexible materials, the angled flaps bend uniformly toward the vehicle if stuck by the ground and return to their original position. The device described herein is easily packaged and shipped, can be easily mounted on the undercarriage of trailers and trucks of different geometries and construction without modification, requires no welding or drilling in the undercarriage when mounting, resists damage, and damaged areas can be easily repaired with replacement parts. The device described herein does not enclose the undercarriage of the trailer like past designs, thereby preventing snow and ice build-up and allowing convenient access to parts underneath the trailer or truck. Unlike past designs, the device described herein also improves road safety by absorbing significant impact in the event of a side collision. Absorbing side impact helps prevent passenger vehicles from entering the area underneath the trailer, which often results in serious accidents because the vehicle is crushed by the trailers rear wheels.